docs: def decompress(self, x, flag):
          """
          calculate the y coordinate given only the x value.
          there are 2 possible solutions, use 'flag' to select.
          """
          x = self.field.value(x)
          ysquare = x ** 3 + self.a * x + self.b
  
          return self.point(x, ysquare.sqrt(flag))